1. Field of the Invention
The invention pertains generally to the art of air conditioning units which utilize supplemental electric resistance heating, and pertains in particular to roof top type air conditioning units in which supplemental electric resistance heat is provided.
2. Description of the Prior Art
The use of electric resistance heat in air conditioning units, including those which are operable as heat pumps, is of course well known as evidenced by the disclosures in U.S. Pat. Nos. 2,934,323, 3,186,477, 3,220,212, 3,529,659 and 3,731,056. For the most part these patents relate to small unit or window type air conditioning units and none of them appear to be concerned with the particular problems of providing frame type electric heating modules in a satisfactory mounting arrangement in a roof top air conditioning unit and under the current constraints relating to power consumption problems. In that connection, a discussion of some of the background problems leading to the invention is considered warranted.
Some of the design considerations pertinent to my invention are set forth in the noted Lackey application and reference should be had thereto for a part of the background.
Electric heat modules, by which is meant a box type frame containing electric resistance heating elements therein and open on both the top and bottom, are readily available in various sizes and capacities such as from the Tuttle Electric Products Division of Emerson Electric Co. It is desirable generally to locate such modules in a position where relatively high velocity air flows over the resistance element so that the modules can be relatively compact and have a high watt density. If a module having a relatively high watt density is located in a position in which the air flow velocity is too low, the heaters will overheat and cut out. However, another consideration is that a relatively high watt density heater will impose a greater resistance to air flow than a relatively low watt density heater occupying the same area transverse to the air flow. The problem of increased resistance to the air flow arising from the use of high watt density heaters in a frame or box can of course be solved by increasing the fan power to get the required air flow. However, in these times in which energy conservation is a prime consideration and the requirements of achieving at least certain specified EER's (which also includes fan power as an element), it is desirable to keep the power consumption of the fans as low as possible.
In the case of the roof top type air conditioning unit disclosed in the noted Lackey patent application, the logical place to place the electric heater modules is tightly up against the discharge openings of the centrifugal fans so that high watt density heater modules may be used. However, within the constraints of the basic roof top air conditioning unit to which the heater elements are to be applied, the resistance to air flow with the modules up tight is sufficiently high that a higher horsepower motor would be required to get the necessary air flow through the indoor air flow section of the unit. The larger fan motor horsepower required would then penalize the EER. A solution to the excessive air flow resistance is to use modules having a larger face area with lower watt density heaters therein and then provide an opening transition duct from the fan discharge to the larger face area modules. While such a move would solve the problem, it imposes its own penalties in that the lower watt density heaters use more wire because of the lower velocity air flow by the heating elements, and also the addition of a transition duct is costly. My invention does not take that approach but does solve the problem even though the relatively high watt density modules are still used.
As will also become apparent from the following material, the invention also provides a mounting arrangement which permits the easy addition of the heater modules to a roof top unit subsequent to its manufacture.